1. Field of the Invention
This invention relates to an ultrasonic motor which rotates a rotor by developing a traveling wave on a stator.
2. Description of the Related Art
Conventionally, an ultrasonic motor which utilizes traveling waves is well known in the art. The conventional motor has a piezoelectric vibrator which is adhered to a stator. The traveling wave is generated on the stator while the piezoelectric vibrator is oscillating. The rotor rotates in response to the traveling wave. For example, the conventional ultrasonic motor is disclosed in Japanese Laid Open Patent Publication No. 58-148682 published on Sept. 3, 1983.
Referring now to FIGS. 7 and 8, a conventional ultrasonic motor is disclosed. A rotor 72 and a stator 73 are contained in the casing or housing 71.
The rotor 72 is connected to the spindle 74 which is rotatably supported in the casing 71. The rotor 72 integrally rotates with spindle 74. A pair of piezoelectric vibrators 75a, 75b are adhered to the stator 73. The piezoelectric vibrators 75a, 75b generate a traveling wave on the stator 73.
A pressure applying mechanism 76 is provided between the casing 71 and the rotor 72. The pressure applying mechanism 76 includes a rotor cam 76a and a spindle cam 76b. The rotor cam 76a is fixed to the rotor 72. The spindle cam 76b is fixed to the spindle 74. Each cam 76a, 76b has a V-shaped bottom. A plurality of steel balls 76c is pinched or secured between the V-shaped bottoms.
The steel balls 76c are positioned at the bottom of the cams 76a, 76b. However, the steel balls 76c move from the V-shaped bottom in response to an increase in load and generate an axial pressure with respect to the spindle 74. Thus, pressure between the rotor 72 and the stator 73 is applied due to the increase of load. The torque which is generated by the rotor is transmitted to the spindle 74 due to the applied pressure.
However, in the conventional ultrasonic motor, the vibration from the piezoelectric elements is confined due to the applied pressure since the pressure applying mechanism 76 generates a large pressure with respect to the increase of the load. Therefore, the conventional ultrasonic motor rapidly reduces rotational speed in response to the increase of the load.